1. Field of the Invention
The invention relates to systems and methods for sterilizing articles that include the use of a gas discharge plasma.
2. Description of the Related Art
Plasmas produced using radio frequency (RF) generators in particular have proven to be valuable tools in processes for the sterilization of medical devices. For example, in U.S. Pat. Nos. 4,643,876 and 4,756,882, which are incorporated by reference herein, Jacobs, et al. disclose using hydrogen peroxide as a precursor in a low temperature sterilization system that employs RF plasma. The combination of hydrogen peroxide vapor and a RF plasma provides an efficient method of sterilizing medical devices without, using or leaving highly toxic materials or forming toxic by-products. Similarly, Jacob, U.S. Pat. No. 5,302,343, and Griffiths, et al., U.S. Pat. No. 5,512,244, teach the use of RF plasmas in a sterilization process.
However, there are problems associated with the use of an RF plasma in a sterilization process. The RF plasma may leave residual hydrogen peroxide on the sterilized article. The residual amount of hydrogen peroxide remaining on the sterilized article depends upon the RF power applied to the article, the amount of time exposed to the RF plasma, and the material of the article. For example, while some plastics (e.g., polyurethane) absorb hydrogen peroxide, other materials (e.g., Teflon) absorb relatively little, thereby yielding less residual hydrogen peroxide after sterilization.
In addition, inherent inefficiencies in the energy conversion from the low frequency (e.g., 60 Hz) line voltage to the RF (e.g., approximately 1 MHz-1 GHz) voltage used to generate the RF plasma limit the power efficiency of such systems to typically less than 50%. Energy efficiency is further reduced by typically 5-20% by virtue of the losses from the required impedance matching network between the RF generator and the load. Such low energy efficiency significantly increases the cost per watt applied to the sterilized articles. The required instrumentation for using RF electrical energy (e.g., RF generator, impedance matching network, monitoring circuitry) is expensive, which also increases the cost per watt applied to the sterilized articles.
One aspect of the present invention is a method of sterilization of an article. The method comprises placing the article in a vacuum chamber and evacuating the vacuum chamber to a predetermined pressure. Gas or vapor species are introduced into the vacuum chamber, and a low frequency plasma is generated within the vacuum chamber, the low frequency plasma having a frequency of from 0 to approximately 200 kHz. The low frequency plasma is maintained for a time period sufficient to substantially remove gas or vapor species from the article.
Another aspect of the present invention is a method of sterilization of an article. The method comprises placing the article in a vacuum chamber and evacuating the vacuum chamber to a predetermined pressure. A low frequency plasma is generated within the vacuum chamber, the low frequency plasma having a frequency of from 0 to approximately 200 kHz. The low frequency plasma is maintained for a time period sufficient to heat the article to aid the evaporation and removal of water and other absorbed gases from the vacuum chamber and the article.
Another aspect of the present invention is a system for sterilizing an article. This system comprises a vacuum chamber coupled to a vacuum pump and a vent, a first electrode, and a second electrode. The system further comprises a first region within the vacuum chamber, the first region comprising a region between the first and second electrodes. The system further comprises a second region within the vacuum chamber, the second region being in fluid communication with the first region. The system further comprises a source of fluid coupled to the vacuum chamber, a process control monitor, and a low frequency power module comprising components adapted to apply a low frequency voltage between the first electrode and second electrode to generate a low frequency plasma in the vacuum chamber, the low frequency voltage having a frequency of from 0 to approximately 200 kHz.